Caption: Rodent ulcer next to ear. Close-up of a scab-like rodent ulcer next to a patient's ear. Rodent ulcers (basal cell carcinomas) are the most common form of skin cancer among fair-skinned people, mostly affecting the elderly. In Australia over half the population has had one by the age of 75, the cause being ultraviolet radiation in sunlight. The tumour normally appears on the face, and starts as a small, flat nodule that grows slowly into the surrounding tissue, breaking down at the centre to form a shallow ulcer. Fortunately, rodent ulcers seldom spread cancer to other parts of the body. Treatment includes surgical removal, cryosurgery (freezing) and radiotherapy.